In an LTE (Long Term Evolution) FDD scheme, one DL (Downlink) carrier and one UL (Uplink) carrier form a pair. An LTE mobile communication system may be operated using a plurality of DL carriers and a plurality of UL carriers, but the correspondence relation between the DL carriers and the UL carriers is one to one.
As illustrated in FIG. 4, a mobile station UE is configured to use only a pair of one DL carrier and one UL carrier at a time point at which the mobile station UE communicates with a radio base station eNB through a radio link.
A mobile station UE in an IDLE (standby) state is configured to select a DL carrier on standby according to a cell selection operation defined in 3GPP TS36.304.
Then, the mobile station UE is configured to specify an UL carrier, which forms a pair with the DL carrier, from broadcast information (specifically, SIB2) in the standby DL carrier.
Herein, on transition from the IDLE state to a CONNECTED (connection) state, the mobile station UE is configured to establish a radio link using the standby DL carrier and the UL carrier forming a pair with the DL carrier.
The radio base station eNB transmits a layer 3 (RRC: Radio Resource Control) message to a mobile station UE in the CONNECTED state to each mobile station UE at a predetermined timing, thereby changing a pair of a DL carrier and an UL carrier for establishing a radio link.
In the LTE scheme, there have been defined an uplink radio resource assignment procedure and a data transmission and reception procedure using PDCCH (Physical Downlink Control Channel) terminated in a layer 1/MAC sub-layer.
Specifically, a radio resource scheduler of the radio base station eNB determines a PUSCH (Physical Uplink Shared Channel) resource of an UL carrier to be assigned to each mobile station UE, and notifies each mobile station UE to be assigned with the PUSCH resource of the determined content through the PDCCH (UL grant) using a DL carrier forming a pair with the UL carrier.
Furthermore, a minimum time unit for assigning a PUSCH resource is a “subframe (TTI: Transmission Time Interval)”, and the radio base station eNB determines a mobile station UE to be assigned with the PUSCH resource in each subframe (TTI), and notifies a corresponding mobile station UE of the determined content using the PDCCH (UL grant).
Furthermore, in a bandwidth of an UL carrier, a minimum frequency unit for assigning the PUSCH resource is “RB (Resource Block)”.
Here, the PDCCH to be transmitted to each mobile station UE is transmitted in such a way that a CRC part of the PDCCH is masked by a unique identifier (C-RNTI) assigned in advance to each mobile station UE.
Specifically, the PDCCH (UL grant) includes information on the sizes and the like of RB to be assigned to a mobile station UE in the subframe (TTI) and TB (Transport Block) to be generated by the mobile station UE.
As illustrated in FIG. 5, if each mobile station UE attempts to detect the PDCCH (UL grant) in a used DL carrier in each subframe (TTI) and succeeds in decoding the PDCCH (detects “CRC: OK”), the mobile station transmits an uplink data signal using a PUSCH resource designated in the PDCCH through a used UL carrier (an UL carrier forming a pair with the DL carrier).
At this time, since the mobile station UE attempts to unmask the CRC part of the PDCCH by the C-RNTI assigned thereto, a result of the unmasking of the CRC makes no sense (“CRC: NG”) in the case of PDCCH assigned to another mobile station UE. Therefore, the mobile station UE determines that PDCCH, a decoding result of which is “CRC: OK”, is assigned thereto.
As illustrated in FIG. 6, since a resource for transmitting the PDCCH through a DL carrier is limited, an event may occur, in which all the PDSCH (Physical Downlink Shared Channel) resources and PUSCH resources may not be assigned due to resource shortage.
Meanwhile, in order to further improve frequency use efficiency, improve peak throughput, and reduce transmission delay in the LTE scheme, an LTE-A scheme has been discussed in the 3GPP and the specification work thereof is under progress.
The main function of the LTE-A scheme is to improve user throughput (CA: Carrier Aggregation) by simultaneously setting a plurality of DL carriers and a plurality of UL carriers in one mobile station UE, and performing communication (refer to Non Patent Literatures 2 and 3).
In addition, the LTE-A scheme has been discussed to be used as the same system while maintaining backward compatibility with the LTE scheme, and even in the case of applying CA as illustrated in FIG. 7, each DL carrier forms a pair with an UL carrier specified from broadcast information (SIB2) in the DL carrier.
As a scenario of the CA, there has been discussed a scenario in which the number of DL carriers and the number of UL carriers simultaneously used in a mobile station UE are asymmetrical to each other.
Particularly, since it is general that the user data traffic in a DL is higher than that in an UL, a scenario, in which the number of DL carriers provided as a system or the number of DL carriers supported as implementation of a mobile station UE is larger than the number of UL carriers, has been emphasized.
In this case, since the number of DL carriers is larger than the number of UL carriers, the relation between a DL carrier and an UL carrier specified from the broadcast information (SIB2) in the DL carrier may not only be N-to-one correspondence but also one-to-one correspondence as illustrated in FIG. 8.